REFRACTORIES 517 This refractory must be fired to the temperature corresponding to cone 17 and cooled quickly so as to produce a dense structure consisting to a large extent of small crystals of siUimanite. The calcine requires to be fired to the same temperature as the body. Insulating Refractories.—For the purpose of preventing the large losses of heat occurring through the walls and arches of furnaces the use of insulating materials applied to the outside or inside of the walls is becoming more and more extensive. The substances used for this purpose are principally kieselguhr or diatomaceous earth or a combination of this material pulverized and mixed with cork dust and bonded by means of plastic clay to form bricks. These refractories are not intended for exposure to high temperatures and cannot be used under such conditions owing to their large contraction in volume and also the low softening point of the kieselguhr-clay combination. The insulating materials of this type serve their purpose admirably and deserve general application for the prevention of the enormous heat losses of industrial practice. Their chief advantage lies, of course, in the low thermal conductivity which is about 0.000344 in c.g.s. units or 1 B.t.u. per hour, per square foot, through 1 in., or about one-sixth of the conductivity of the average fire brick at 200°F. This reduction becomes still more marked at higher temperatures so that at 1,600°F. it may be only one-ninth of the thermal conductivity of the refractories commonly used. The temperature coefficient of these materials is close to unity so that the conductivity remains quite constant until temperatures are reached which bring about a change in structure i.e.f a decrease in the porosity. The light weight of the insulating materials, 25 to 30 Ib. per cubic foot, is also a desirable feature. They are composed practically of silica. In commercial use the materials are marketed in the form of blocks or bricks or as a powder. For uses where resistance to higher temperatures is unnecessary and where some of the insulating quality may be sacrificed compositions consisting of approximately 49 per cent of cork (20 to 60 mesh), 16 of plastic fireclay, 22 of Georgia or Florida kaolin and 13 of crushed firebrick, 4 to 16 mesh, may prove useful. In using heat-insulating materials it is necessary to realize that insulation necessarily raises the mean temperature of the wall between the surface exposed to the heat and the insulation to a point far above that applying to ordinary conditions. The damming up of the heat thus raises especially the temperature of the surface of the firebrick. It is inevitable therefore that insulation requires the use of materials which are more heat resisting. Many instances are on record where firebrick have failed soon after insulation was applied. Heat insulation may thus be said to have increased the necessity for superior refractories. BIBLIOGRAPHY Transactions of the American Ceramic Society. Journal of the American Ceramic Society. Transactions of the Ceramic Society (British). Bruno Kerl, "Handbuch der Gesammten Thonwaaren Industrie". Carl Bischof, "Die Feuerfesten Tone". Albert Granger, "Fabrication et emploi des mate'riaux et produits r6fractaires utilises dans 1'Industrie". Le Chatelier, "La silice et les silicates". E. Bourry, "The Ceramic Industries". A. B. Searle, "Modern Brick Manufacture". JBureau of Standards, Technologic Papers Nos. 7, 10, 116 and 144.